In the past, the image display devices disclosed in Japanese Patent Application Kokai No. 2000-352689 and Japanese Patent Application Kokai No. 2001-264682, for example, have been known as so-called see-through type head mounted image display devices (head mounted displays) which allow the user to view a display image superimposed on a view of the outside world while observing the conditions of this outside world.
Furthermore, Japanese Patent Application Kokai No. 2001-264682 discloses not only a see-through type head mounted image display device, but also an image display device that is not used as a see-through type device (i.e., that conducts only light from an image forming element to the eye of the user without superimposing other light from the outside world or the like on this light from the image forming element) while having substantially the same construction as such a see-through type device; an example in which this image display device is contained in the flipper part of a portable telephone is also disclosed.
In these image display devices, a reduction in size and weight is achieved by using a reflective type holographic optical element. Such a reflective type holographic optical element is superior in terms of wavelength selectivity, and can selectively diffract/reflect only light in an extremely limited wavelength region. Accordingly, in cases where a see-through type image display device is constructed, the amount of light that is lost when light from the outside world or the like passes through the image combiner can be noticeably reduced by using a reflective type holographic optical element.
Furthermore, in these image display devices, since a reflective type hologram is used as the image combiner, the reflective type hologram is disposed so that this hologram is embedded in respect to the optical path in order to separate light from the outside world and from the display light. Moreover, in these image display devices, a liquid crystal display element is generally used as the image forming element in order to achieve a reduction in size and weight, and an LED, which is a compact and inexpensive light source, is used as the light source that illuminates this image forming element.
However, in these conventional image display devices, although a good display image can be viewed in cases where the center of the pupil of the eye of the user and the center of the exit pupil of the image combiner coincide, the display screen gradually becomes darker as the center of the pupil of the eye of the user moves toward the periphery of the exit pupil of the image combiner from the center of this exit pupil, and the display image appears to be blurred, so that the image is not always adequate in terms of quality. Furthermore, during actual use, it can frequently happen that the center of the pupil of the eye of the user deviates from the center of the exit pupil of the image combiner.